After moving all mailboxes from a mailbox database to other mailbox databases (both in Exchange 2016), including archive and arbitration mailboxes the mailbox database still cannot be deleted. Trying to remove the mailbox database still generates the following error message:
This mailbox database contains one or more mailboxes, mailbox plans, archive mailboxes, public folder mailboxes or arbitration mailboxes, Audit mailboxes. To get a list of all mailboxes in this database, run the command Get-Mailbox -Database <Database ID>. To get a list of all mailbox plans in this database, run the command Get-MailboxPlan.
Get-Mailbox -Arbitration or Get-Mailbox -Archive doesn’t return anything.
Exchange 2016 introduced a new type of mailbox, the auditlog mailbox. This can be returned by the Get-Mailbox -Auditlog command:
Move this auditlog mailbox to another mailbox database and the old mailbox is now fully empty and can be deleted.